In recent years, a greater emphasis is placed on startup characteristics of an intermittent oscillator circuit to improve a battery life. In an IC-chip circuitry system, a constant of equivalent series impedance is becoming smaller as a piezoelectric vibrator is increasingly miniaturized, often deteriorating oscillation startup characteristics. These disadvantages are more noticeable particularly in the technical fields relevant to smart meter-based remote monitoring, automated meter readers, and security systems such as radio-equipped security and disaster control devices.
FIG. 19 illustrates a conventional oscillator circuit equipped with a piezoelectric vibrator. In the drawing, 10 is a piezoelectric vibrator, C1 is a first load capacitance, C2 is a second load capacitance, Inv is an inverter for amplification, and RF is a feedback resistor for setting a suitable direct current bias point on the input side of the inverter Inv. In the oscillator circuit, an amplifier device which amplifies an oscillation output of the piezoelectric vibrator 1 has an inverter in a single stage. FIG. 20 is an illustration of gain-frequency characteristics.
To ensure good startup characteristics demanded in an intermittent operation, a large driving performance is required. The startup characteristics of the oscillator circuit depend on a negative resistance of the oscillator circuit. The negative resistance increases as the driving performance is larger, improving the startup characteristics (less startup time). However, such a large driving performance leads to an increase of current consumption at the same time, failing to meet the demand for improvement of a battery life.
FIG. 21 illustrates a conventional oscillator circuit in which an improvement is made (see the Patent Document 1). Capacitance elements C1 and C2 are connected to between both ends of a piezoelectric vibrator 10 and ground GND, and a feedback resistor RF, and inverters in three stages (IV0, IR1, IR2) are connected in parallel to between the both ends of the piezoelectric vibrator 10. The inverter IV0 is an always-ON inverter in the first stage, the inverter IR1 is an ON/OFF inverter in the second stage, and the inverter IR2 is an ON/OFF inverter in the third stage. An input terminal of the ON/OFF inverter IR1 in the second stage and an output terminal of the ON/OFF inverter IR2 in the third stage are connected to each other by a selector switch 21. A timer circuit 100 is provided to time-control the selector switch 21 and connected to a control terminal of the selector switch 21 by way of the inverters IR1 and IR2 in the two stages. Power terminals of the ON/OFF inverter IR1 in the second stage and the ON/OFF inverter IR2 in the third stage are connected to a power source by way of a switching transistor Qx. An output of the timer circuit 100 is also outputted to a gate of the switching transistor Qx. OUT is an oscillation output terminal, and 20 is an amplifier device which amplifies an oscillation output of the piezoelectric vibrator 10.
In an initial phase of oscillation startup, the selector switch 21 is OFF, and the switching transistor Qx is ON, and the ON/OFF inverter IR1 in the second stage and the ON/OFF inverter IR2 in the third stage are operational, constituting a three-stage inverter unit with the always-ON inverter IV0 in the first stage.
When a given period of time passed after the oscillation started, the timer circuit 100 times out and outputs a switchover control signal Sc, in response to which the selector switch 21 is switched to ON by way of the inverters IR1 and IR2 in the two stages. Further, the switching transistor Qx is disconnected, and the ON/OFF inverters IR1 and IR2 in the second and third stages thereby short-circuited are disconnected from a path and become non-operational. As a result, the driving performance depends on the always-ON inverter IV0 alone in the post-startup phase where the oscillation is stabilized.
Thus, the oscillator circuit illustrated in FIG. 21 starts the operation using the inverters in multiple stages having good startup characteristics, assuring a high negative resistance in the initial phase of oscillation startup, and then switches to the single-inverter operation in the post-startup phase where the oscillation is stabilized to obtain the oscillation with less noise.
The gain of the single-inverter oscillator circuit of FIG. 19 is very low as illustrated in FIG. 20, whereas the oscillator circuit of FIG. 21 driven by the inverters in three stages has a very large gain (see a characteristic curve illustrated on the upper side of FIG. 7).
There is an increasing demand in recent years for an intermittent operation of TCXO (Temperature-Compensated Crystal Oscillator) to improve a battery life of mobile telephones, and startup characteristics are becoming a very important factor to meet the demand. The intermittent operation can save electricity, and high startup characteristics can start the operation sooner to avoid any disadvantages associated with the intermittent operation. When an oscillator circuit used in a mobile telephone is designed to the specifications of an intermittent operation, for example, quick startup makes it unnecessary to take any waiting time, enabling a quick system startup and a longer battery life.